Tracked vehicles such as snowmobiles and snow groomers drivingly engage the ground through one or more endless tracks. Endless tracks conventionally include an outer side with a pattern of projecting lugs (or ribs) that are designed to engage the snow or other ground surface, apply traction, and propel the vehicle. Conventional endless tracks also include an inner side that engages one or more drive sprockets, which, in turn, are operatively connected to a propulsion system of the vehicle. The drive sprockets and the inner side of the endless track typically include mating teeth that provide traction between the drive sprockets and the endless track. Conventional drive sprockets use either external (radially extending) or internal (axially extending) teeth or both.
FIG. 1 illustrates a conventional arrangement of four drive sprockets 1050, 1060. Two drive sprockets 1050 are to be disposed on outer sides of an endless track 1080, and two drive sprockets 1060 are to be disposed on a central portion of the endless track 1080. The drive sprockets 1050 include a plurality of axially-extending sprocket teeth 1070 (9 teeth) on one side thereof, while the drive sprockets 1060 include a plurality of axially-extending sprocket teeth 1070 (9 teeth) on each side thereof. In some cases, the sprockets 1050 or 1060 also have radially extending teeth. In other cases, the drive sprockets 1050, 1060 have 8 or 10 sprocket teeth 1070. The drive sprockets 1050 are used in combination with the drive sprockets 1060 to ensure sufficient application of the engine torque to the endless track 1080 without allowing ratcheting (rotation of the sprocket without equivalent rotation of the track). Only two sprockets 1050 or 1060 could also be used, if the sprockets 1050 or 1060 provided sufficient application of the engine torque to the endless track 1080.
The endless track 1080 includes a plurality of longitudinally-spaced internal track lugs (or teeth) 1090 projecting inwardly from an inner side of the endless track 1080 for contacting the drive sprockets 1050, 1060. The sprocket teeth 1070 engage the internal track lugs 1090 to provide traction between the sprocket 1050, 1060 and the endless track 1080. Alignment cleats 1085 are laterally offset from the track lugs 1090 on the endless track 1080 and the sprocket teeth 1070. The internal track lugs 1090 are disposed so as to form two outer rows 1091 and four inner rows 1092, in a longitudinal direction 1002. The rows 1091 and 1092 are disposed adjacent to each other in a lateral direction 1004, and the internal track lugs 1090 are aligned in the lateral direction 1004. A pitch 1093 between the track lugs 1090 is constant. The pitch 1093 is measured in the longitudinal direction 1002. The track 1080 also features two longitudinal rows of apertures or windows 1030. The track 1080 includes a plurality of external track lugs 1095. The external track lugs 1095 are distributed in lateral rows, and the rows are disposed at a pitch 1097 in the longitudinal direction 1002. The pitch 1097 of the external track lugs 1095 equals the pitch 1093 of the internal track lugs 1090.
While the above endless track and sprocket assembly functions to provide torque transfer while avoiding ratcheting, a greater number of sprockets is required when the sprockets do not have radially extending teeth or there is a greater amount of torque to be transferred to the track than can be applied by the sprocket assembly without ratcheting. On the other hand, when using sprockets with radially extending teeth, these sprockets are aligned with the windows of the track, and travel over the alignment cleats 1085 over which the slide rails glide. The contact between the alignment cleats 1085 and the sprocket causes undesired noise and vibration. It also requires the slide rails to be shortened due to the space required for the sprockets.